1. Field of the Invention
This invention relates to a plasma display panel, and more particularly to a method of setting an average picture level that is adaptive for preventing a brightness inversion phenomenon and a method of driving a plasma display panel using the same.
2. Description of the Related Art
A plasma display panel PDP is a display device using a phenomenon that visible ray is generated from a fluorescent substance when ultraviolet ray generated by gas discharge excites the fluorescent substance. The PDP is thinner and lighter than a cathode ray tube CRT, which has been used as main display means so far, and can be embodied of high definition and wide screen.
Referring FIG. 1, a discharge cell of a three-electrodes AC surface discharge PDP includes a first electrode Y and a second electrode Z formed on an upper substrate 1, and an address electrode X formed on a lower substrate 4.
The address electrode perpendicularly intersects a pair of sustain electrodes each including either the first electrode Y or the second electrode Z.
There are a dielectric layer 2 and a protective film 3 deposited on the upper substrate to cover the first electrode Y and the second electrode Z.
There is a dielectric layer 5 deposited on the entire surface of the lower substrate 4 to cover the address electrode X and there are barrier ribs 6 formed parallel to the address electrode X on top of it.
There is inactive mixture gas as discharge gas interposed into a discharge space of a discharge cell provided between the upper/lower substrates 1 and 4 and the barrier ribs 6.
In order to realize gray level of a picture, there is a frame (field) driven by being divided into several sub fields that have different light-emission frequency. Each sub field is divided into a reset interval (or initialization interval) for initializing cells of a whole screen, an address interval for selecting the cell and a sustain interval for realizing the gray level in accordance with a discharge frequency, then to be driven.
FIG. 2 illustrates a driving apparatus of a conventional plasma display panel.
Referring to FIG. 2, the conventional driving apparatus of the PDP includes a first reverse gamma corrector 10A connected between an input line 9 and the PDP 26, a gain controller 12, an error diffuser 14, a sub-field mapping unit 16 and a data aligner 18; a frame memory 20 connected between the input line 9 and the PDP 26, a second reverse gamma corrector 10B, an average picture level APL controller 22 and a waveform generator 24.
The first and the second reverse gamma corrector 10A and 10B applies reverse gamma correction to a gamma corrected video signal to linearly convert the brightness value depending on the gray scale value of a video signal. The frame memory 20 stores the data R,G,B of one frame portion and supplies the stored data to the second reverse gamma corrector 10B.
The APL controller 22 receives the video data corrected by the second reverse gamma corrector 10B to generate N (N is an integer) step signals for controlling the number of sustaining pulses. The gain controller 12 amplifies the corrected video data from the first reverse gamma corrector 10A as much as effective gain.
The error diffuser 14 diffuses an error component of a cell to the adjacent cells to finely control the brightness value. The sub-field mapping unit 16 re-allots the video data corrected from the error diffuser 14 by sub-fields.
The data aligner 18 converts the video data inputted from the sub-field mapping unit 16 to be suitable for the resolution format of the PDP 26, and then supplies to an address driving integrated circuit IC of the PDP 26.
The waveform generator 24 generates a timing control signal by the inputted N step signal from the APL controller 22 and supplies the generated timing control signal to the address driving IC, a scan driving IC and a sustain driving IC of the PDP 26.
In this way, the APL controller 22 of the driving apparatus of the conventional plasma display panel is used to emphasize the area that is relatively bright when the luminosity of the whole image is dark.
There is an operation process of the APL controller 22 explained in detail in reference to FIG. 3.
Referring to FIG. 3, the number of sustain pulses decreases as the step of APL gets higher. In other words, when the APL step is below a threshold APLth (to be set approximately between 17 to 24), the number of sustain pulses is set to be the maximum sustain number (approximately between 800 to 1200). When the APL step is over the threshold APLth, the number of sustain pulses gradually diminishes. Here, when the APL step is maximal, the number of sustain pulses is set to be the minimum sustain number (approximately near 200).
On the other hand, power consumption increases in proportion to APL when the APL is below the threshold APLth, and is uniformly maintained on the whole when the APL is over the threshold APLth. That is, the APL controller 22 of the conventional PDP is used in order to emphasize the relatively bright area when the luminosity of the whole image is dark and to have the power consumption maintained uniformly.
On the other hand, the number of sustain pulses in accordance with APL steps is determined by the following Equation 1.Nsus=1/(a+b×x)  [Equation 1]
Herein, x represents a current APL step, Nsus denotes the number of sustain pulses. And, a and b can be determined from a maximum sustain number, an APL threshold Ath, a minimum sustain number and a maximum APL step.
For instance, a linear equation in relation to a and b can be calculated by setting the maximum sustain number to be 1200 (Nsus=1200) and the threshold APLth of APL to be 17 (x=17). Further, quadratic equation can be calculated by setting the minimum sustain number to be 182 (Nsus=182) and the maximum APL step to be 255 (x=255). If the value of a and b is calculated in use of the linear equation and the quadratic equation, a has the value of 4.9×10−4 and b has the value of 1.96×10−5.
At this moment, by the Equation 1, the number of sustain pulses Nsus is set to be 680.2721 . . . if the APL is 50, and the number of sustain pulses Nsus is set to be 408.1632 . . . if the APL is 100. Further, the number of sustain pulses Nsus is set to be 226.7573 if the APL is 200. In other words, the conventional APL controller 22 determines the number of sustain pulses by rounding off a fraction below a decimal point in use of Equation 1.
There is explained how to derive Equation 1 in reference to FIG. 4.
Referring to FIG. 4, a conventional equivalent circuit of a PDP includes a first capacitors Cd formed between the first Y and second Z electrodes and the address electrode X respectively, a second capacitor Cg formed by a gap between the first electrode Y and the second electrode Z, a third capacitor Cdi formed by the dielectric layers 2 and 5, a fourth capacitor formed by a plasma and a inactive gas, a first resistor formed by the resistance value of the plasma, a second resistor Rd formed by a data driver and a third resistor Re formed by a scan driver.
In such an equivalent circuit of a conventional PDP, a first electric power P1 consumed in a panel is proportional to the multiplication of the number of sustain and a current APL step (P1∝Nsus×x). Further, a reactive power P2 that charges the capacitors Cd, Cg, Cdi and Cv formed in the panel is proportional to the number of sustain (P2∝Nsus).
A second electric power P3 consumed in an energy recovering device for recovering the electric power supplied to the first and second electrodes is proportional to the reactive power P2 (P3∝P2∝Nsus). A third electric power P4 consumed by internal resistance in a power supplier that supplies electric power to the panel and the energy recovering device is proportional to the sum of the first electric power P1 and the second electric power P3 (P4∝P1+P3∝Nsus(K+Kx)).
Accordingly, the consumed power in the whole PDP is defined as Equation 2.Ptot∝P1+P3+P4∝Nsus(a+b×x)  [Equation 2]
At this moment, an average brightness of a screen is determined by Equation 3.B∝Nsus×x  [Equation 3]
Even if the APL step changes in Equation 2, a maximum sustain number needed for sustaining power consumption uniformly is as the followings.Nsus(x)=k/(a+bx)  [Equation 4]
Herein, Equation 1 is derived since k is set to be 1. On the other hand, when the maximum number of sustain is 1200 (Nsus=1200) and the threshold APLth of the APL is 17 (x=17), if the brightness of B(x)−B(x−1)-herein, x is a natural number of 1 or more-is calculated in use of Equation 3, it can be shown as in FIG. 5.
In other words, FIG. 5 is a graph represented by subtracting the brightness value of a previous APL step from the brightness value of a current APL step.
Referring to FIG. 5, it can be seen that the brightness is inverted when setting the number of sustain pulses when using a conventional Equation 1. In other words, there is shown an area where the brightness is inverted (where values are below ‘0’ in a Y axis) when the APL step is over 120. Such a brightness inversion phenomenon takes place by rounding off a fraction below a decimal point when calculating the number of sustaining pulses with Equation 1. On the other hand, if the brightness is inverted in the PDP, flicker phenomenon may occur, thereby causing picture quality deterioration.